In the web offset printing press business, there is a common problem in printing successive images of different colors in perfect register from unit to unit. This problem is due to the unstable character of the printing paper, which tends to increase in width or fan out, as it passes through the successive printing press units.
Thus, the paper web passes through the first printing unit and is printed with a first color image. The passage of the paper web through the first printing unit causes the web to fan out, or be increased a small amount in width, so that the width of the first printed image is also increased. When the web passes through the second printing unit, the first printed image is no longer in perfect register with the second printed image, produced by the second printing unit. This fanning out or increased width of the printing web tends to occur as the web passes through each of the printing units, and then travels to the next printing unit. This problem of web fanout, or increased web width, due to the printing action of the printing press units, makes it impossible to achieve perfect crosswise or lateral registration between the successively printed color images in a multicolor printing press.
In the past, somewhat primative attempts have been made to deal with this problem of web fanout. One approach has been to install a wheel between each pair of printing units. The wheel is brought into contact with the web, to apply a variable amount of pressure between the wheel and the web, so as to form a longitudinal bustle or ridge in the web. The bustling pressure may be applied at various points on the web, including the center of the web and other points across the web. The bustling of the printed web will actually pull the web back to its original width, so that the first printed color image is drawn back into register with the next printed color image.
The bustling wheel has generally been made of steel or nylon and has been mounted on a threaded rod, so that the wheel can be screw adjusted, up or down, to apply pressure on the printed web. The use of the bustle wheel has the disadvantage that it can smear the ink or produce tracking marks on the printed web, unless the wheel can be brought into engagement with a clear unprinted longitudinal area on the printed web.
To avoid the ink smearing problem caused by bustle wheels, there has been another somewhat primative approach, to use a piece of copper tubing or pipe, with or without some type of nozzle, to blow compressed air up against the web, between successive printing units, to force the bustling action without actually touching the printed paper web.